Copper(II) complexes with cyclo(L-aspartyl-L-aspartyl) and cyclo(L-glutamyl-L-glutamyl) derivatives and their antioxidant properties

Two new functionalized cyclodipeptides have been synthesized with the aim of obtaining a good model of superoxide dismutase. Better to mimic the: active site of this metalloenzyme, these two compounds have been designed to allow a great co-ordination flexibility. Copper(II) complexes with cyclo(-L-aspartyl-L-aspartyl-) or cyclo(-L-glutamyl-L-glutamyl-)bis(histamine) (L) have been thermodynamically and spectroscopically characterized and their antioxidant activity tested against enzymatically generated O-2(-). Taking into account the Speciation of the system, it has been determined that the more active species against O-2(-) is the [CuL(2)](2+) complex. Electron spin resonance measurements suggest for this species the presence of four imidazole nitrogen atoms in a slightly tetrahedrally distorted co-ordination plane. The [CuLH(-2)] complex species also possesses four-nitrogen co-ordination involving two deprotonated peptide nitrogen atoms. The complex [CuL(2)](2+) showed the highest antioxidant activity and reasons for this behaviour are proposed on the basis of spectroscopic and voltammetric data.